1. Field of the Invention
This invention relates to improvements in a photoelectric microscope which is employed for the measurement of a very small distance. More particularly, it relates to a photoelectric microscope which is so improved that the position of a reference line (or mark) provided on the surface of a test piece (sample) can be measured with high precision as a distance from the optic axis of an optical system for measurement.
2. Description of the Prior Art
It is well known that photoelectric microscopes of various types are employed for the purpose of measuring very small distances in the micron to submicron order. For example, the photoelectric microscope of the vibration slit type acquires a signal indicative of the position of a reference line, that is, indicative of the distance from the optic axis of a measuring optical system to the reference line, in such a way that the image of the reference line provided on the surface of a test piece is formed on a vibrating slit plate by an objective. That light passing through the slit plate is then detected by a photoelectric transducer which is disposed behind the slit plate, and the detected signal is synchronously rectified by a driving signal for the vibrating slit plate.
In the prior art apparatus of this sort, however, when the vibration center of the vibrating slit plate deviates or when the distribution of the intensity of illumination around the reference line on the test piece is not a perfect line-symmetric distribution, a measurement error arises. When the contrast of the reference line on the test piece changes, the detection sensitivity fluctuates, resulting in a further measurement error.